This invention relates generally to the non-invasive measurement of blood pressure and, more particularly, to the measurement of blood pressure via an automated blood pressure apparatus.
The measurement of blood pressure is a common procedure used in hospitals, clinics and physicians' offices as a tool to assist in diagnosis of illness and monitoring of sick patients, as well as an indicator of the general status of a person's health. In standard non-invasive blood pressure measurement practice, blood pressure is measured using an inflatable sleeve, commonly referred to as a cuff, to measure arterial blood pressure. The cuff, which is adapted to fit around a limb over an artery of a patient, typically around the patient's upper arm over the brachial artery, includes an interior chamber adapted to be inflated with air to provide pressure on the artery.
Electronic blood pressure measurement devices for automatically inflating the cuff and automatically sensing the blood pressure either during inflation of the cuff or during deflation of the cuff are well-known in the art. In such devices, a motor driven pump is operatively connected to the interior chamber of the cuff by means of a tube, often referred to as a lumen. Upon activation of the pump motor, air is pumped by the pump through the tube to inflate the interior chamber of the cuff to a pressure sufficient to stop the blood flow through the artery. A bleed valve is also operatively connected in fluid communication with the interior chamber to permit depressuring of the interior chamber when it is desired to deflate the cuff, either step-wise or rapidly, as desired. Generally, a pressure sensing device, typically a pressure transducer, is operatively connected in fluid communication with the interior chamber of the cuff to directly sense the pressure within the interior chamber of the cuff.
Automated blood pressure measurement devices commonly employ either an ausculatory technique or an oscillometric technique to detect when the systolic blood pressure, which corresponds to the cessation of blood flow through the artery, is reached, and when the diastolic blood pressures, which corresponds to unrestricted blood flow through the artery, is reached. In a conventional ausculatory method, a sound sensing device, commonly a microphone, is provided in operative association with the cuff to listen for pulsating sounds, known as Korotkoff sounds, associated with the flow of blood through an artery under pressure. In a conventional oscillometric approach, one or more pressure sensing devices, for example pressure transducers, are provided in operative association with the cuff to detect small oscillatory pressures that occur within the cuff as the result of the pulsating characteristic of blood flow through the artery.
Electronic circuitry, including a central processing unit, is provided that processes the signals from the cuff pressure sensor, and, if present, the microphone or additional pressure sensors, and determines the systolic and diastolic blood pressures. Typically, a digital display is also provided for displaying the systolic and diastolic blood pressures. The signals indicative of the systolic and diastolic blood pressure measurements may be transmitted to an external device, such as a laptop or a patient monitor, for display and/or data recording.
Automated blood measurement devices may be either two-lumen or single lumen devices. In a two-lumen apparatus, the first lumen provides a conduit connecting the inflation chamber of the cuff in fluid communication with the pump and the second lumen provides a conduit connecting the inflation chamber of the cuff in fluid communication with a pressure transducer, or other pressure sensing device. Therefore, the chamber is inflated during the inflation period by the pump passing air flow through the first lumen, while the pressure within the cuff is monitored independently through a static second lumen, unaffected by the flow of air through the first lumen. In a single lumen device, however, the inflation chamber of the cuff is connected in fluid communication with both the pump and the pressure sensor through the conduit of single lumen. Consequently, on a single lumen device, the pressure sensed by the pressure sensor will be impacted by the pressure losses experienced by the air flowing through the first lumen. Therefore, at any given instant in the inflation process, the pressure sensed by the pressure sensor will be greater than the actual cuff pressure by an amount equal to the sum of the pressure losses experienced by the air being pumped through the first lumen.
The overall pressure loss encountered in a single lumen may attributed to a number of factors including the amount of airflow (i.e. the air flow rate), the resistance to flow through the tube itself, which varies dependent upon tube length and cross-sectional flow area, and the pressure drop through the connectors at the respective ends of the tube, with the air flow rate being the dominant determinant of pressure loss. The amount of air flow required to maintain a consistent pressure rise rate during the inflation of the cuff is variable and dependent upon a number of factors, including, for example, pump drive voltage, pump efficiency, cuff size, tightness of the cuff wrap about the patient's limb, limb stiffness, and cuff pressure. Given the number and complexity of these variables, it is not practical to attempt to calculate the actual pressure loss at a given point in the inflation process. Therefore, automated apparatus designed to measure the systolic and diastolic blood pressures during the inflation process, rather than during a step deflation process initiated only after full inflation of the cuff, use a two-lumen configuration. However, due to the convenience of and the patient comfort associated with the “inflation BP” technique, it would be desirable to have a method of accurately correcting the sensed cuff pressure to enable use of the “inflation BP” technique on a single-lumen blood pressure measurement apparatus. The accuracy standard for BP measurement established by the Association for the Advancement of Medical Instrumentation is +/− 3 millimeters Hg (three millimeters Mercury).
A detailed discussion and description of the operation of an exemplary embodiment of an electronic apparatus for the non-invasive measurement of blood pressure is presented in the aforementioned U.S. Patent Application Publication No. US2005/0033188A1.
U.S. Pat. No. 5,632,278 discloses a device for automatically measuring blood pressure using a sphygmomanometer cuff connected to a pressure generator and a pressure sensor via a common tube wherein the blood pressure measurements are made during a controlled deflation of the cuff.
U.S. Patent Application Publication No. US2004/0127801A1 discloses a noninvasive sphygmomanometer wherein blood pressure measurements, sensed during slow deflation of the sphygmomanometer cuff, are subsequently corrected based on a value of the arm circumference length calculated during the cuff inflation process based on a relationship between a pressure in the bladder of the cuff and the measured discharge amount of the pressure pump inflating the bladder.
European Patent Application No. EP0775465A1 discloses an automatic sphygmomanometer wherein if a first blood value that is determined while a pressure of the cuff is slowly increased is adjudged abnormal, a second blood pressure value is determined while the cuff pressure is decreased.